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Secondary DNS and Primary DNS

We have our product server located in Singapore and I have installed and
configured DNS server to act this server as secondary name server for our
product.

Our primary DNS server for this product is located in India.
I have defined this new DNS server as a secondary DNS server for my domain
(abc.in) on rediff.com.
I have specified IP address over there in name-server mapping settings.
It is got reflected as well.

If I see by #whois abc.in, I am getting the changes in DNS servers. It is
showing secondary DNS server properly.

I have seen this by various DNS checker tools as well.

I have tested following things and all are worked fine.
On the new server 53 udp/tcp is in open mode.
dig is also working
zones are also getting updated automatically by primary DNS server located
in India.

But my problem is that the Secondary DNS server is not resolving my product
server (www.www.abc.in) if the primary DNS server is down.

Why secondary DNS server is not resolving my product server?

It should resolve the query in the failure of primary DNS. It should work as
soon as possible.

Still the problem is there. I have changed the slave DNS server as a host
name instead of IP address on rediff.com and tested by by making primary
DNS down and for some time (few min) I was getting access to
http://abc.inbut after some time it failed. So I have made primary
server on and then it started to work.

I was testing it on a laptop with Internet card and before testing I had
used following command to flush DNS c:/ipconfig/flushdns

Still this problem has not been resolved.
I have tested the secondary DNS server by following way.

1. telnet <nameserver> 53 --------- It is working
2. by using dig command ----------It is working
# dig <mydomain name> @secondary name server
3. I have seen 'whois <mydomain>' this is also showing both the DNS
servers.
4. I have seen some online DNS checking services and these services are
also showing positive results.

But if I check it by making primary DNS server down, it is not working.
People are not getting access to our product server (http://abc.in)

I have already kept the named.conf files on both the servers on this RT
request.

Can anybody tell me if there are any other ways to check the secondary DNS
server?
So can anybody help me regrading this?